1. Field of the Invention
The invention relates to a transducer for generating optical contrasts in the near-field representation of topographies of an object by outcoupling evanescent waves from the underside of a transducer, the transducer underside being arranged in the focal plane of an imaging optical system assigned to the transducer.
2. Related Art
It is known from U.S. Pat. No. 5,349,443 that a plane-parallel glass plate (transducer) in the immediate vicinity of a specimen surface can be used to generate a tunnel contrast. For this purpose, the transducer is illuminated obliquely and in the process the critical angle of total reflection between glass and air is exceeded. As a result, surface waves, known as evanescent waves, occur at the transducer underside. Illumination and observation of the transducer underside are performed with the aid of an immersion objective of high numerical aperture. Object points which dip into the propagation region of the evanescent waves outcouple the light at these points. On the transducer underside, the topography of the object stands out as spatial intensity modulation of the light.
Since the field strength of the evanescent wave field decays exponentially with distance from the transducer underside and is already still only I/e of the original,       after    ⁢          xe2x80x83        ⁢    δ    =            λ      ⁢              xe2x80x83            ⁢      o              2      ⁢              xe2x80x83            ⁢      π      ⁢                        NA          -          1                    
where xcex0=vacuum wavelength; and
NA=numerical aperture of the objective, the sample surface to be imaged must be brought up to the transducer as far as a fraction of a micrometer. This condition creates problems in several regards.
In addition to the fine surface topography relevant to imaging, inside the object field to be observed the object generally has a surface curvature which can be observed at a relatively large scale. It is thereby frequently difficult, if not impossible, to approach sufficiently close to the points to be observed. Even approaching an ideally flat specimen is very difficult, if not impossible, whenever there is a particle of dirt on it. Moreover, adjusting the flat transducer underside to be sufficiently parallel to the specimen surface is attended by a high outlay.
U.S. Pat. No. 5,349,443, mentioned above, discloses using a flexible transducer. The transducer is to be stiff and flat enough inside the object field, but is to be able to yield to the unevennesses in the specimen surface over relatively large distances. This flexible transducer is intended to be able to compensate even for dirt particles.
Such a flexible transducer constitutes an improvement over a rigid large-area glass plate. However, it must be accepted that the transducer touches the specimen surface locally in order to be able to adapt itself to the specimen surface over a large area. For the purpose of investigating sensitive specimens such as, for example, integrated circuits, this is not at all acceptable, however, because touching the specimen generally entails destroying it. Moreover, a higher degree of transducer flexibility is traded off against a reduction in the hardness of the transducer material, with the result that the underside can easily be scratched and therefore becomes unusable for outcoupling evanescent rays.
Instead of an immersion object with a transducer, U.S. Pat. No. 5 5,004,307 discloses using a dry objective, in which a vitreous spherical segment is arranged in the space between the objective and object. This segment is denoted as a xe2x80x9cSolid Immersion Lens (SIL)xe2x80x9d. This SIL is such that the side facing the object has a flat face. The light incident from the dry objective is totally reflected at this face, which also represents the focal plane of the objective, with the result that tunnel contrast can arise. An advantage of this arrangement resides in that it is generally possible to approach the specimen because of the special shape of the SIL. The disadvantages of this device reside, however, in the very expensive production of the SIL, in the very high outlay on adjustment, in the production of an extremely flat plane surface through polishing with a very slight surface roughness, in the imaging errors caused by the SIL in conjunction with higher numerical apertures and, finally, also in the high outlay in exchanging the SIL if the lower plane surface should be damaged by touching the specimen.
In the near-field operating mode, both the evanescent wave field immediately outside the SIL plane surface and the light wave field propagating in air are used to image an object which is arranged in the vicinity of the SIL. The light wave field can be used for focussing, and the evanescent wave field enhances the resolution in air by a factor of 1/n, n being the refractive index of the material from which the SIL is produced. Consequently, it is possible, at least partially, to avoid diffraction effects which limit the resolution for imaging in-air in standard microscopy by means of the near-field operating mode.
U.S. Pat.No. 5,121,256 also discloses using this effect to produce topographies on an object surface. There is a permanent need in optical lithography to transfer increasingly smaller pattern structures from a photomask onto a photoresist.
In view of the foregoing, it is an object of the invention to provide a transducer for producing optical contrasts in the near-field region of the transducer, with the aid of which it is possible freely to approach the specimen without making contact, which can be easily exchanged, which can be adapted to specific specimen geometries, which can be displaced perpendicular to the optical axis of the imaging optical system, and whose production costs are favorable.
In accomplishing some or all of the preceding objects, there has been provided according to the present invention a transducer in an imaging optical system for generating optical contrasts in the near-field representation of topographies of an object by outcoupling evanescent waves from the underside of the transducer. The transducer comprises a substrate having a transparent plane-parallel protuberance corresponding to the field size of the imaging optical system and pointing toward the object. The transducer outcouples evanescent waves from an underside of the transducer, where the transducer underside is arranged in a focal plane of the imaging optical system.
One of the advantages achieved with the transducer according to certain embodiments of the invention is that, with the protuberance of the flat plate in the middle of the object field, it is necessary to bring only a small area of the plate underside close to the surface of the specimen. Thus, unevenness over large areas of the specimen, or even dirt particles outside the protuberance, do not prevent the approach. This form of the transducer also permits the imaging optical system and the transducer to be joined as a mechanical unit, and renders it possible for the objects to be investigated to be moved under this unit.
Further objects, features and advantages of the invention will become apparent from the detailed description of the preferred embodiments of the invention that follows, when considered together with the accompanying drawings, which are in part schematic.